1. Field of the Invention
The present invention relates to a zoom lens system which is suitably used for an electronic camera such as a video camera or a digital still camera, a film camera, a broadcasting camera, or the like, and an image pickup apparatus using the same.
2. Related Background Art
In recent years, along with high function promotion and miniaturization of an optical apparatus using a solid-state image pickup element, such as a video camera, a digital still camera, portable terminal equipment (PDA: personal digital assistants), a mobile telephone, and a conventional photographing camera, it is required that high optical performance is compatible with miniaturization also in an optical system used for such an optical apparatus.
In particular, for a zoom lens used for those optical apparatuses, efforts are made on increase in a high zooming ratio, shortening of a total lens length, miniaturization of a front lens diameter, simplification of a lens configuration, and the like. In addition, along with the high performance promotion (digitization and high definition (HD) promotion) of a video cassette recorder, the high image quality promotion has progressed.
As one method for realizing the high quality promotion, there is a method in which color separation of an image is carried out by a color separation optical system and the image of a subject is captured by every color using solid-state image pickup elements corresponding to the individual colors. In addition, as one method for miniaturizing an optical system, there is a method adopting a so-called rear focusing system in which lens units other than a first lens unit on a side of an object are moved to focus (e.g., refer to JP 6-51199 A,    JP H6-337353 A (corresponding to U.S. Pat. No. 5,530,592),    JP H6-347697 A (corresponding to U.S. Pat. No. 5,583,698),    JP H7-199069 A (corresponding to U.S. Pat. No. 5,583,698),    JP H7-270684 A (corresponding to U.S. Pat. No. 5,963,378),    JP H7-318804 A (corresponding to U.S. Pat. No. 5,963,378),    JP H9-281390 A (corresponding to U.S. Pat. No. 6,178,049),    JP H9-281391 A (corresponding to U.S. Pat. No. 6,178,049),    JP H9-304698 A (corresponding to U.S. Pat. No. 6,178,049),    JP H11-23965 A (corresponding to U.S. Pat. No. 6,084,722),    JP H11-287952 A, JP H11-287954 A, JP 2000-305016 A, and JP 2003-241088 A).
In general, since the relatively compact and light lens units are moved during zooming, the rear focusing system has the features that the compatibility with the auto-focusing system is excellent and so forth because a small driving force for driving the lens units is only required, and the focusing can be speedily carried out. In addition, the rear focusing system has the features that the proximity photographing, especially, very proximity photographing is possible.
In the zoom lens proposed in each of the above-mentioned patent applications other than JP 2000-305016 A, the long back focus is ensured in which it is supposed to dispose three color separation prisms on an image side. However, in each of the embodiments described therein, the zooming ratio is equal to or smaller than 16 times.
On the other hand, along with the high density promotion of the solid-state image pickup element such as a CCD, a high resolution frequency is required for the zoom lens. In general, the image degradation due to the diffraction is promoted as a stop diameter of a stop provided in an optical system becomes small, or as a stop becomes opening state in which an opening shape is widely different from a true circular shape.
Adoption of an iris stop or insertion of an ND filter into an optical path are known as a method for solving the problem, in which the stop diameter is prevented from decreasing, thereby suppressing an influence of diffraction to a minimum. However, according to this method, the optical system has a tendency to be easily scaled up due to complexity of a stop mechanism and an increase in an interval on an optical axis required for the insertion of the ND filter.
For example, in the case of the zoom lens proposed in JP 2000-305016 A, it is hard to say that the intervals of the front and the rear the stop are sufficiently ensured.